This invention relates to the thermomechanical processing of titanium alloys, and, more particularly, to an approach for attaining a highly textured structure after mechanical working.
Pure metals and metallic alloys solidify with their atoms arranged in highly ordered arrays that are regular and repeating. These arrays, known as the crystallographic structure of the metal, are maintained over large, macroscopic dimensions of the metal piece. For example, the atoms of an alloy may be visualized as lying at the corners and the body center of a cube, producing a "body centered cubic" or BCC crystallography. In another example, the atoms may be visualized as lying in a repeating hexagonal array, producing an "hexagonal close packed" or HCP crystallography. (There are a number of other common types of crystallography as well.) The crystallography of a metallic alloy may be characterized in terms of the type of crystallography (e.g., BCC or HCP) and the orientation in space of the crystallographic unit (e.g., a cube with its faces oriented in particular directions).
Some metals may be composed entirely of only one type of crystallographic structure, which is of the same orientation in space throughout, and such metals are termed "single crystals". In most structural applications, it is preferable to have present contiguous small islands or "grains", each of which has its own crystallographic type and crystallographic orientation in space. The individual grains may each be of the same crystallographic type, or several different types may be present in the same material due to the compositional and processing characteristics of the alloy.
The individual grains may have random crystallographic orientations in space, or they may have a tendency to have their crystallographic directions aligned to some degree. The latter situation is termed a "texture". It is known that particular textures can be beneficial in structural alloys, because the textures produce good combinations of strength, ductility, creep, and fatigue properties. For alloys wherein the properties are dependent upon the texture, the control of texture provides an important way of improving the mechanical properties of the metals.
Many of the properties of metallic alloys can be understood in terms of their crystallographic types and orientations, and the interrelationships of the grains within a metallic piece. For example, if a metal of a selected composition is provided in different crystallographic types, grain orientations, and grain sizes, the resulting properties of the metallic pieces are altogether different. The crystallographic theory of metals is used to relate the properties to these structural parameters. Conversely, once the basic understanding of the relationship between the crystallographic parameters and the metallic properties is attained, then various techniques may be used to select the best properties and further engineer the materials to achieve even better properites.
The development of metallic alloys for use in some of the most demanding aerospace and other applications involves these types of investigations. As an example, titanium alloys are used in portions of aircraft engines and structures because titanium has excellent properties at temperatures of up to about 600 C., and can be processed to attain particularly good mechanical and other types of properties. There is a good fundamental understanding of the relationship of crystallographic characteristics of the titanium alloys to their properties.
However, in some cases, the understanding of metallic properties has outpaced the ability to actually manufacture metals having selected types of properties. Combinations of desirable material properties are sometimes difficult to achieve, and therefore approaches to attaining those properties through careful selection of alloying elements and processing are necessary. The present invention deals with the selection of titanium alloys and their processing to achieve a desirable crystallographic texture.
By way of background, titanium alloys can be classified as alpha phase alloys, beta phase alloys, and alpha-beta phase alloys. Alpha phase alloys have the hexagonal phase crystallography at room temperature, and change to the beta phase crystallography only at very high temperature. The beta phase transforms to alpha phase upon cooling, and there is little beta phase left at room temperature. Beta phase alloys have the beta phase crystallography at room temperature, and retain this structure upon heating and cooling. Alpha-beta alloys are similar to the alpha phase alloys, but actually exhibit both alpha and beta phases at room temperature because the beta phase can be stabilized to exist at room temperature along with the alpha phase.
It is desirable in many cases to process alpha or alpha-beta phase titanium alloys by first heating them into the fully beta phase, working the alloy in the beta phase, and thereafter cooling the alloy. The working of large pieces requires less power when they are hot, and the large prior beta grains produced by this approach lead to good properties in the resulting alloy. Unfortunately, it has been observed that the crystallographic texture produced by working the titanium alloy in the beta phase range is close to random. There has been proposed no approach for achieving textured structures of such materials.
There exists a need for a method of controlling the crystallographic texture of titanium alloys worked in the beta phase range. Such an approach should be compatible with existing working processes, and should permit retention of other desirable characteristics of the titanium alloy. The present invention fulfills this need, and further provides related advantages.